Machines used in construction sites and other off-road locations generally experience loss of traction. Moreover, four wheel drive machines used in these locations also experience traction loss. For example, slipping occurs to either the front wheels or rear wheels, or to all four wheels.
For example, a machine typically used on construction sites is a wheel loader. Wheel loaders commonly have four driven wheels and are often articulated. As is well known, an articulated machine includes front and rear body parts hinged together by an articulation/joint for relative movement about a ..horizontal.!. .Iadd.vertical .Iaddend.axis. Each body part includes a wheel set. When one of the body parts move relative to the other the machine turns. During normal operation a wheel loader will experience wheel-slip to all four wheels, especially when loading.
To alleviate such problems, various mechanical anti-spin methods have been developed and placed in commercial use. Such methods have been proven to have various problems, especially during cornering of a machine. For example, one method to prevent front or rear wheel-slip is by locking the differentials. However, since the differential operation is restrained, cornering ability is greatly deteriorated.
An alternative approach involves the provision of separately actuatable drive wheel brakes. An operator selectively applies a braking force to the spinning or slipping wheel, and effects a balancing of power through the differential mechanism. The application of the braking force to the slipping wheel simulates increased traction and results in a more even distribution of power between the differentially driven wheels. This approach is commonly used on farm machines.
However, one problem not addressed in preventing wheel-slip is accounting for the articulation angle (the angle representing the relative movement of one body part to the other). The relative movement of one body part to the other provides for machine cornering. When cornering the rotational velocities between the machine's radially inner and outer wheels are different; consequently, the radially outer wheels have a faster rotational velocity than the radially inner wheels.
Some methods fail to accommodate the normal wheel speed differential which arises during the machine cornering. For example, when the machine is cornering such methods may brake the faster rotating wheel (radially outer wheel) causing excessive tire wear due to the radially outer wheel or wheels dragging.
Other methods may drive only the slower wheel in a turn, making the machine hard to steer and applying excessive torque to the wheel being driven, often causing failure of the final drive.
The present invention is directed to overcoming one or more of the problems as set forth above.